192 - MARINE INVERTEBRATES 
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group. They are plentiful everywhere, occurring between tide- 
marks and at great depths on the floor of the sea. Rocks pro- 
tected from the sun are often incrusted with the calcareous forms, 
while branching species hang from the rocks and adorn rock 
pools. Membranipora, in lace-like sheets, will be found on the 
larger seaweeds as well as on various submerged objects. The 
beautiful little Crisia may be looked for on the fronds of red 
alge, and Alcyonidium, in soft moss-like patches, on Fucus. On 
some beaches are scattered the sea-mats, Flustra foliacea, looking 
like bits of brown paper. A pocket-lens will reveal the marvel- 
ous and orderly arrangement of the two layers of cells, or zoccia, 
placed back to back. There is not a more surprising object in 
the animal kingdom than a living branch of Bugula. The cells 
on its spiral, profusely branched clusters have appendages re- 
sembling a bird’s head, with its bill constantly opening and shut- 
ting. Of the erect polyzoans some are entirely calcareous and 
rigid; such species live in deep water, and their colonies are 
called corallines, as are also the calcareous alge. They have ex- 
isted in vast numbers since early geologic times, and are plentiful 
in the Tertiary deposits, known as Coralline Crags. Other erect 
forms have a caleareous framework, which is not continuous, but 
has horny intervals, so that the colonies are flexible and can be 
swayed by the tides in shallow water where they live. Others 
have complete horny exoskeletons. The same differences occur 
in the incrusting forms, those on the seaweeds being either flex- 
ible, so that they bend with the plant, or if entirely calcareous 
are so small that they do not break when the plant sways to and 
fro. Still others are gelatinous and resemble bits of sponge. 
The calcareous forms are usually ornamented with ridges which 
form beautiful patterns. 
The individuals of the polyzoan colonies are small polyp-like 
organisms which have a cup-like inclosure of horny or of calca- 
reous substance, or of the two combined, as mentioned above. 
This inclosing sheath is the exterior or cuticle of the animal and 
is called the zowciwm, the whole colony being called the zoarium. 
The animal substance within the zocecium is the polypide, and 
consists of the mouth, tentacles, and alimentary system. A disk 
at the anterior end of the body, bearing the circle of tentacles, is 
